FIG. 1 illustrates signal communication apparatus 100 for sending a signal from a transmitter 121 across to a receiver 122. Apparatus 100 might be included in a computer or other such electronic equipment. The transmitter 121 is located in a first field replaceable unit (FRU) 111, while the receiver 122 is located in a second FRU 112. Each FRU typically comprises a printed circuit board (PCB). FRU1 111 and FRU2 112 are joined by connector C1 11 on FRU1 111 and connector C2 12 on FRU2 112. The transmitter 121 is attached by solder ball 121A to line 1 101, which in turn is linked to connector C1 11 by solder ball 11A. Receiver 122 is linked by solder ball 122A to line 2 102, which in turn is linked to connector C2 12 by solder ball 12A. Accordingly, a signal from transmitter 121 passes via solder ball 121A, line1 101, solder ball 11A, connectors C1 11 and C2 12, solder ball 12A, line2 102, and then solder ball 122A before arriving at receiver 122. This route from transmitter 121 to receiver 122 will be referred to as transmission path 110.
A fault may occur in apparatus 100, and this can lead to a signal from transmitter 121 failing to arrive at receiver 122. Although such a fault could develop anywhere along the transmission path 110, for example due to a crack in a PCB trace corresponding to line1 101 or line2 102, in practice the most probable location of such a fault is in one of the solder balls 121A, 11A, 12A, 122A, or at the connection between connector C1 11 and connector C2 12.
A short circuit fault anywhere along the transmission path 110 may cause the signal to be absent along the whole of path 110. An open circuit fault anywhere along the transmission path 110 causes the signal to be absent between the fault and the receiver. Practical experience has shown that most faults on PCB transmission paths and connectors are open circuits.
In a situation where transmitter 121, receiver 122, and the whole of transmission path 110 are all located on a single FRU, then the service strategy should a communication fault be detected is relatively straightforward. It is known that the fault lies in the FRU containing both the transmitter and the receiver, and accordingly a service engineer can replace this FRU with a properly functioning counterpart.
However, for the configuration shown in FIG. 1, it is difficult to know whether a fault has arisen in FRU1 111 or in FRU2 112. One possibility of course is to replace both FRU1 111 and also FRU2 112, but this is expensive, since it involves replacing one FRU that is presumably still fully functional. Alternatively a service engineer may try to replace first FRU1 111 and then, if this does not rectify the fault, replace FRU2 112 (as well as potentially re-installing the original, presumably non-faulty, FRU1 111). However, this requires the service engineer to have both FRU1 and FRU2 available. In addition, the service strategy is more time-consuming than replacement of a single FRU, and may require two or more interruptions of normal machine operations to complete the repair. These problems are exacerbated if the transmission path 110 extends across more than two FRUs.
It is therefore desirable to be able to investigate transmission path 110 in more detail to try to locate a fault to a particular FRU (e.g. one of FRU1 or FRU2), thereby allowing the repair to be performed with the replacement of just a single FRU. It is especially helpful if the location of a fault to a single FRU can be performed automatically, so that a service engineer only needs to be provided with a replacement for the particular FRU known to be faulty, rather having to be provided with potential replacements for both (all) of the FRUs involved in the transmission path.
Nevertheless, the ability to locate a fault to a particular region within a single FRU can also be of benefit. For example, a FRU may contain multiple automatic system reconfiguration units (ASRUs), which can be deconfigured on an individual basis pending replacement of the complete FRU. Accordingly, the determination of the location of a fault within a FRU may help a system to decide which particular ASRU(s) to deconfigure in response to the fault, for example to ensure that the fault is not exercised pending replacement of the entire FRU.
One possible approach to locating a fault on a PCB interconnect such as shown in FIG. 1 is to add a trace that branches from line1 101 to a first detector, and likewise a trace that branches from line2 102 to a second detector. These extra traces could be used to monitor the signals on line1 101 and line2 102, and hence to try to locate any fault within transmission path 110. However, this approach suffers from the problem that the extra traces may adversely impact the transmission line characteristics of line1 101 and line2 102. Moreover, it may be hard to accommodate the extra traces within the PCB layout of the relevant FRU. It is especially difficult to accommodate such extra traces within an existing circuit design.
U.S. Pat. No. 6,714,021 describes an alternative approach based on time domain reflectometry for locating a fault on a PCB interconnect. This approach involves transmitting a signal of known shape along a transmission wire and looking at any reflections that are generated as a result of the transmission due a mismatch in impedance. Thus, in a typical circuit, such as shown in FIG. 1, there may be a reflection from various locations, such as from the interface between line1 101 and connector C1 11, from the junction between connector C1 11 and connector C2 12, and from the termination of line2 112 at the receiver 122. These reflections from the different locations will be separated in time, based on the signal propagation time from the transmitter 121 to the point of reflection and back again.
Any circuit fault in transmission path 110 will generally alter the pattern of reflected signals. For example, a fault is likely to introduce its own signal reflection, while at the same time it will generally suppress reflections from locations beyond the fault. Accordingly, an analysis of signal reflections received back at transmitter 121 from transmission path 110 can help to determine the presence and location of any fault in the transmission path 110.
One drawback with time domain reflectometry is the need for appropriate circuitry to be incorporated into transmitter 121 to generate the test signal and to analyse the reflected signal. This will tend to increase the cost and complexity of the design, especially as PCBs become more and more complicated, with potentially multiple transmitters on a single FRU.